The present invention relates to radio frequency signal communication systems. Specifically, a filter structure is disclosed for use in direct analog to digital conversion of a radio frequency signal which also decimates the recovered digital signal.
In wireless communication such as cellular telephones base station transceivers, it is possible to directly convert a received radio frequency signal from an analog signal to a multi-bit digital signal. In carrying out the conversion process, a delta-sigma analog to digital converter oversamples the incoming radio frequency signal to provide a high-speed oversampled digital bit stream. The high-speed (oversampled) bit stream is further filtered in a narrow band digital low pass, or a band pass, decimation filter which decimates the high-speed oversampled digital signal to generate a multi-bit low noise, nyquist rate digital signal.
Using the sigma-delta analog to digital converter, it is possible to shape the noise so that the noise is minimized about the carrier frequency of the radio frequency signal Fc which is being converted to a digital signal. The sigma-delta analog to digital conversion process provides oversampling at an oversampling rate Fs, which is N times higher than the carrier frequency signal Fc being received. The sampling frequency is also set to be higher than two times the bandwidth (BW) of the signal which is being converted to a digital signal. Accordingly, Fs=Nxc3x97Fc and OSR (Oversampling Rate)=Fs/2xc3x97Bw.
The foregoing process of creating a high speed oversampled bit stream, and filtering and decimating the high bit filter stream to provide a nyquist rate digital signal requires significantly complex hardware.
As an approach to reducing the hardware requirements, Schreier and Snellgrove have proposed, in the paper, xe2x80x9cDecimation for Bandpass Sigma-Delta Analog to Digital Conversionxe2x80x9d, IEEE International Symposium on Circuits and Systems, 1990, pgs. 1801-1804, a system which can provide decimation for a bandpass sigma-delta analog to digital conversion process which minimizes the hardware requirements. A sigma-delta modulator is provided, having either a low pass or a bandpass response by selecting an appropriate error transfer function. A complex modulation and complex filtering system is disclosed to filter and decimate the high speed data sequence produced by the sigma-delta modulator.
While the foregoing system reduces the hardware complexity necessary to carry out these later stages of processing, it still requires modulating the high speed digital signal by four sine and cosine sequences to produce a baseband signal, and then individually filtering and decimating the resulting modulated signals. Accordingly, it is an object of this invention to further reduce the circuit complexity for filtering and decimating a high speed bit stream produced by a delta-sigma modulator.
In accordance with the invention, a decimation filter is provided for delta-sigma analog to digital converters. The decimation filter design includes a baseband conversion signal which is related to the modulator sampling frequency Fs. In order to reduce the circuit complexities for filtering and decimating the high speed digital signal, the system uses a sampling signal Fs which is substantially 6Fc where Fc is the carrier frequency of interest.
In selecting Fs=6Fc, baseband conversion is provided by a complex modulator which is designed to multiply the high speed digital sequence by SIN(xcfx890nT) and COS (xcfx890nT) n=0, 1, 2, . . . ; T=1/Fs. By selecting the modulation frequency of xcfx890=xcfx80/3, a sequence of modulating signals is provided for recovering a real and imaginary baseband signal comprising a very simple structure, wherein only two signal paths are needed for processing the real and imaginary components. Each of the real and imaginary components are further filtered to derive the decimated nyquist frequency signal.